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GI Parasitism in the Intermountain West

Less is known about gastro-intestinal parasitism in the western US. A study was undertaken to provide baseline data on gastro-intestinal parasite prevalence, species composition, and anthelmintic resistance on sheep farms that use irrigated or sub-irritated pastures in the Intermountain West. Producers were also surveyed regarding their management strategies and production parameters.

Sampling occurred in the summers of 2017 to 2019 on twenty-five sheep operations in Montana (n=15), Wyoming (n=9) and Utah (n=1). Sampling was limited to operations with a history of parasite challenge. Samples were composited into a single sample for coproculture and LDA analysis (DrenchRite® test).

Across all operations, the minimum, median, and maximum composite EPG were were 100, 550, and 11050 epg, respectively. Haemonchus contortus was the most commonly identified worm species across all operations (68.5%), followed by Trichostrongylus (12.4%), Oesphagostumum (8.9%), Teladorsagia (8.4%), and Cooperia (1.8%). H. contortus represented more than 50% of the larvae identified on 18/25 of the operations (72%).

Twelve farms were evaluated for dewormer resistance. Resistance to H. contortus was highly prevalent with benzimidazoles (91.7%), followed by ivermectin (50%) and moxidectin (8%). Assays were unable to quantify resistance to levamisole.

Results from the present study indicate that anthelmintic resistance to multiple drug classes is a concern in Intermountain West flocks that routinely utilize irrigated pastures as a forage base. Survey results indicated that knowledge and adoption of sustainable parasite control methods is lacking.

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